Bonjour Chat Initiation

I have a small network of mac computers. We obviously have Bonjour. Everything works great except for one guy can not initiate an IM using iChat, Adium, what ever.

Everything else works fine. File sharing is fine. He shows up fine on my buddy list. I can IM him, and he can respond just fine. The one single thing he can not do is start an IM to anyone else. (I take that back, he can initiate chat instances to Windows computers on the Bonjour network - there's only a couple).

It's so bizarre. I can't figure it out. I don't see how every other thing about our network and Bonjour can work for everyone except for this one single issue.